1. Field of the Invention
The invention relates to the purification of elemental yellow phosphorus (e.g., white phosphorus).
2. Relevant Art
Elemental yellow phosphorus, which is produced from crude phosphate by electrothermal means is generally found to contain arsenic and/or other organic contaminants, generally polynuclear aromatics in varying amounts, which account somewhat for its opaque and yellow appearance. Purified or distilled phosphorus has a transparent light yellow or white appearance. The impurities are undesirable because they are often found to restrict the use of secondary phosphorus products (e.g., phosphoric acid salts and organic phosphorus compounds). Yellow phosphorus is one of several allotropic forms of phosphorus.
It is known that elementary yellow phosphorus is a useful starting material for the manufacture of phosphorus derivatives, such as P.sub.2 S.sub.5, for example. To this end, it is often necessary to first free the phosphorus from contaminates contained therein. The purification of phosphorus by subjecting it to treatment with concentrated sulfuric acid in a reactor has been described. Sometimes, however, the reactor may be found to show the phenomena of local overheating. This in turn can initiate self-ignition of the phosphorus and culminate in fire outbreaks and explosions.
It is also known that yellow phosphorus can be purified by treating the phosphorus with between 0.5 and 5 weight percent of an aqueous suspension of activated carbon. It has further been disclosed to purify yellow phosphorus with from 0.5 to 5 weight percent of an aqueous suspension of activated carbon containing between 50 and 500 kilograms per cubic meter of activated carbon.
It has also been disclosed in German Pat. No. 1,143,794 to Cremer to purify phosphorus by subjecting it to treatment with polyphosphoric acid (density 1.91 to 2.08; 75% to 85% P.sub.2 O.sub.5) followed by filtration through a bed of activated charcoal and ignited Fuller's earth.
The process as disclosed in the above referenced German patent, entails mixing the polyphosphoric acid with the liquid unpurified phosphorus and thereafter separating the two layers either by initially drawing off the lower polyphosphoric acid layer, or by diluting the spent polyphosphoric acid with water until it is separated as an upper layer so that the phosphorus can be drawn off as the lower layer. Either of these methods of recovering the purified phosphorus is time consuming and otherwise complicates a simple and direct purification process.